Do more research. Asking one person will only get you that person's preference - Ask lots more people and look for patterns in the results.
The RNIB (UK organisation for blind and partially sighted people) recommends yellow text on a black background but this may not sit well with your particular users.
Not only do you have to think about contrast but you have to think about colour combinations too. Colour blindness affects around 4.5% of the population of the world - That's 9 in every 200 users. Luckily there are tools like the Colour Contrast Analyzer to help you out with that.
Also, vision is not the only area to consider accessibility. If you have small controls that are placed close together that's going to be difficult for people with muscular or motor problems who find it difficult to control a mouse easily. Some people cannot use a mouse at all - does your product allow for keyboard controls? Does your product rely on audio cues? If so then you'll need a visual equivalent for deaf people.
Finally, the most tricky thing to cover is accessibility for people with cognitive difficulties: does your product make sense? Is there too much information presented in a disorganised way or is it broken into easily understandable chunks? Have you cut out all of the unnecessary form fields and other junk?
I made a low-level simulator if you want a small glimpse into the world of someone struggling with accessibility issues.